Ray structures on Teichm\"uller Space

Abstract

While there may be many Thurston metric geodesics between a pair of points in Teichm\"uller space, we find that by imposing an additional energy minimization constraint on the geodesics, thought of as limits of harmonic map rays, we select a unique Thurston geodesic through those points. Extending the target surface to the Thurston boundary yields, for each point $Y$ in Teichm\"uller space, an \enquote{exponential map} of rays from that point $Y$ onto Teichm\"uller space with visual boundary the Thurston boundary of Teichm\"uller space. We first depict harmonic map ray structures on Teichm\"uller space as a geometric transition between Teichm\"uller ray structures and Thurston geodesic ray structures. In particular, by appropriately degenerating the source of a harmonic map between hyperbolic surfaces (along \enquote{harmonic map dual rays}), the harmonic map rays through the target converge to a Thurston geodesic; by appropriately degenerating the target of the harmonic map, those harmonic map dual rays through the domain converge to Teichm\"uller geodesics. We then extend this transition to one from Teichm\"uller disks through Hopf differential disks to stretch-earthquake disks. These results apply to surfaces with boundary, resolving a question on stretch maps between such surfaces.